The present invention relates to a new type of ceramic membrane akin to (micro)fibers for microfiltration and ultrafiltration of fluids and for filtration and separation of gases, and to a method of manufacturing this membrane.
Membranes having a (micro)fiber geometry, in other words a tubular geometry with an outside diameter of up to 3 mm and an inside diameter comprised between 400 and 2000 xcexcm are already known and available commercially. They are made of organic materials and have, because of this, the well-known advantages and disadvantages of these materials. Thus, if the mechanical characteristics of these (micro)fibers, and in particular their flexibility, allow them to be mounted in a filtration module, they nevertheless suffer from low chemical and heat resistance which limits their use to a temperature below 70xc2x0 C. and a pH range comprised between about 4 and about 10.
Additionally, ceramic (micro)fibers already have been proposed. However, in all cases, these inorganic (micro)fibers typically adopt the conventional geometry of organic fibers. Moreover, the ceramic fibers currently known in the art may be useful for filtration, particularly due to their homogeneity, but these fibers typically are constituted of a single porous medium, acting both as the support (the fiber is self-supporting) and the filtering medium. This is unlike conventional systems of a larger size, which comprise a macroporous support coated with a porous filtering layer.
The current geometry of ceramic fibers currently known in the art nevertheless results in their having low mechanical strength. Additionally, manufacture and use of the ceramic fibers modules are difficult because the fibers have a tendency to break for different reasons, such as a sudden shock or pump vibration.
There thus exists a need for (micro)fibers which are strong and allow ready assembly into a module.
The aim of the present invention is thus to provide a multi-channel porous ceramic fiber.
This fiber corresponds to a porous ceramic bar which is perforated by several channels. The bar preferably is a porous ceramic material having a porous structure (in the conventional sense of the term) and a variable porosity. The axis of said channels preferably are parallel to the axis of the ceramic bar.
This fiber is constituted of a single porous medium, acting both as the support (the fiber is self-supporting) and the filtering medium. The channels that are crossing the fiber are exiting at each side of the fiber; these channels are non-communicating with the permeate side, except through their porosity. All these channels perform the same task (separating, reacting, etc.); none is used to transport permeate (or the result of fluid having crossed the wall of another channel.
According to one embodiment, the rank of the channels is at the maximum 2, i.e. one central channel and channels disposed around approximately according to a circle. The rank can also be only 1, in which case the channels are disposed approximately according to a circle.
According to one embodiment, the channels are distributed at the vertices of a regular polygon, the order of which is comprised between 3 and 6. In addition, a supplementary channel may occupy the center of the said polygon where the order is greater than 3, the order being preferably 5 or 6.
The fiber and/or the channels can have any suitable shape, for example a circular cross-section, a channel cross-sections in the shape of orange quarters, or other suitable shapes are possible. The same can apply to the fiber cross-section; a circular geometry can be replaced by a multi-lobe geometry or the like. In the case of an orange-quarter geometry (or where a channel is not circular), the diameter of such a channel will be defined as the diameter of a circular channel having the same cross-section. Where the fiber does not have a circular cross-section, the diameter of such a fiber is similarly defined as the diameter of a circular fiber having the same cross-section.
In accordance with a preferred embodiment, the fiber and/or the channels have a circular cross-section.
Preferably again, all the channels are substantially identical; this is a preferred way of limiting pressure drop and throughput differences from one channel to another along the fiber.
According to one embodiment, the fiber according to the invention has the following characteristics:
(i) a channel diameter comprised between about 150 and about 2000 xcexcm, and more preferably between about 300 and about 1,000 xcexcm; and/or
(ii) an envelope ratio Re corresponding to the ratio of porous ceramic fiber diameter to channel diameter such that Re is comprised between about 2.5 and about 15, preferably between 4 and 10; and/or
(iii) a fill ratio Ro corresponding to a ratio of the sum of channel cross-sections to porous ceramic fiber cross-section such that Ro is comprised between about 0.03 and about 0.45, preferably between about 0.04 and about 0.35 and advantageously between about 0.15 and about 0.35; and/or
(iv) a sustain ratio Rs corresponding to a ratio between mean wall thickness measured along the radius of a fiber and the diameter of a channel passed through, said mean thickness corresponding to the mean of channel wall thickness located on a radius of said fiber passing through a maximum number of channels, such that Rs is comprised between about 0.3 and about 2.5, preferably between about 0.5 and about 1.5; and/or
(v) a thickness ratio Rp corresponding to the ratio between channel wall thicknesses along a radius of the fiber passing through a maximum number of channels, such that Rp is comprised between about ⅓ and about 3, preferably between about xc2xd and about 2, thickness ratio Rp being advantageously about 1. According to another embodiment, this ratio is between about 2 and about 3.
Fiber diameter can extend up to, for example, about 25 mm, preferably up to about 15 mm; typically this diameter is comprised between about 2 and about 10 mm, preferably between about 3 and about 7 mm.
The fibers according to the invention have a crush resistance that is higher than that of the conventional (micro)fiber geometry. Thus, for equal filtration surfaces et/ou equal filtration flowrates, one gets the following breaking strength: for a monocanal fiber from 0,1 to 1N, for a fiber having four channels from 25 to 35N, for a fiber having seven channels from 60 to 100N.